Communication systems are designed to transfer information between two devices over a medium in the presence of disturbing influences. Intersymbol interference (ISI) is one well-known disturbing influence in which transmitted symbols become elongated and interfere with adjacently transmitted symbols. This spreading of symbols is generally caused by the non-ideal frequency response associated with all communication mediums. Because ISI has a similar effect as noise, communication is made less reliable.
One of the most basic solutions for mitigating the effects of ISI is slowing down the speed at which symbols are transmitted over the medium. More specifically, the transmission speed can be slowed down such that a symbol is only transmitted after allowing previously transmitted symbol pulses to dissipate. Although slowing down the speed at which symbols are transmitted can eliminate the effects of ISI, it is generally an unacceptable solution for many of today's communication applications. In fact, many of today's communication applications require speeds in the multi-gigabit per second range. At such high speeds, ISI can completely overwhelm a signal transmitted over even a few inches of printed circuit board trace, a few feet of copper cable, or a few tens of meters of optical fiber.
Therefore, a filtering process, referred to as equalization, is often used to flatten the frequency response of a communication medium to mitigate the effects of ISI. Equalization can be applied at the receiver and/or at the transmitter using a precoding technique. Precoding uses channel state information of the communication medium to pre-equalize symbols before they are transmitted to compensate for the expected ISI from one or more earlier transmitted symbols.
An issue with precoding is that the channel state information generally must be derived at the receiver and then fed back to the transmitter (due, for example, to part of the overall channel equalizer still being implemented at the receiver), and many communication standards do not provide a mechanism to continually perform this process. Instead, the channel state information is usually only derived once at the receiver and then fed back to the transmitter at startup, before application-level data is exchanged between the two devices. Because ISI can change over time with temperature, positioning, and the impedance of the communication medium, for example, the inability to continually adapt the precoder at the transmitter can lead to an increased bit error rate at the receiver.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.